The proton spin has also been in a crisis in the era of the
constituent quark model.
The European Muon Collaboration (EMC) experiment found that
the baryon spin is not only due to the spins of
the valence quark [1]}.
It has been commonly assumed that the proton's spin=1/2
was formed by two quarks with parallel spins and
the third quark with opposite spin.
In the EMC experiment, a quark of a polarized proton target was struck
by a polarized muon beam, and the quark's instantaneous spin was measured.
It was expected that the spin of two of the three quarks would cancel out
and the spin of the third quark would be polarized in the direction of the
proton's spin. Thus, the sum of the quarks' spin was expected to be
equal to the proton's spin.
Surprisingly, it was found that the number of quarks with spin in the
proton's spin direction was almost the same as the number of quarks
whose spin was in the opposite direction.
Similar results have been obtained in many experiments afterwards,
demonstrating clearly that both generalized parton distributions
and transverse momentum distributions are important
in the nucleon spin structure [2]}.
Our modern understanding is that the nucleon spin comes
not only from quarks but also from gluons,
and various contributions can be calculated using
e.g. Ji's sum rule [3]}.
The abnormal magnetic moment of proton (much larger than
that for a Dirac point-like particle) is generally considered as
an indication of a more complicated internal structure
than simply three spin=1/2 quarks in a relative \(S\) -wave.
